This disclosure generally relates to equipment and methods for non-destructive inspection, and more particularly relates to methods and apparatus for inspecting structures having irregular and variable shapes, especially soft-tooled structures made of composite material.
A variety of elongated composite structures may have relatively confined internal cavities that require inspection in order to assure that the structure meets production and/or performance specifications. Conventional composite structure cured with hard tooling results in composite radii that are well defined and repeatable. In contrast, the composite radii formed using soft tooling are not always well defined and may vary from part to part. In some cases, dimensional or contour variations may be greater than those that would result from using hard tooling. These larger variations make reliable inspection using conventional methods more challenging. In view of the deviation from circularity of soft-tooled composite radii, the term “radiused surface” as used hereinafter should be construed non-strictly to include surfaces that vary from being a section of a circular cylinder.
Critical composite structure in aerospace and potentially in applications outside aerospace must be inspected to required specifications to ensure structural integrity. Inspecting soft-tooled composite structures presents distinct yet interrelated challenges. Critical inspection areas include the radiused surfaces of composite part joint fillets. Moreover, such soft-tooled surfaces must be inspected in a production environment. For a production inspection, the inspection rate must be sufficient to meet the part production rate.
For ultrasonic inspection of composite structure, the ultrasound beam should ideally enter at 90 degrees to the local surface of the composite part being inspected. If it does not enter at 90 degrees, it will be refracted off normal and a return echo from any possible internal structure or anomaly will not be optimum. Traditionally a 90-degree entry angle is maintained by holding a sensor array at a precisely fixed position in space relative to the surface. While this works well for known surfaces, such as flat or cylindrical surfaces of a given, fixed radius and circular shape, this approach will not provide adequate results for surfaces which are, for example, parabolic, irregular, or of varying radius of not necessarily cylindrical cross section. Traditional methods of interrogating such a radius with ultrasound fail to keep the sound path sufficiently perpendicular over the entire inspection area.
Current practice in inspecting composite part joint fillets consists of sliding a probe that is geometrically aligned to a fixed fillet radius along the joint. If the radius changes away from this fixed alignment, the data becomes unusable. To realign to the new geometry, a physically different probe can be substituted or an adaptable probe can be manually readjusted. The readjustment or complete replacement, of the ultrasonic probe(s) during the operation slows the inspection process time dramatically.
There is a need for an automated solution to the problem of inspecting composite parts with radii that vary along the length of the part in which the sensor energy enters the composite part volume very close to the local perpendicular at the inspection site.